首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   563546篇
  免费   46291篇
  国内免费   1022篇
  2018年   17274篇
  2017年   16029篇
  2016年   14027篇
  2015年   8471篇
  2014年   9358篇
  2013年   13713篇
  2012年   19444篇
  2011年   29345篇
  2010年   23470篇
  2009年   18869篇
  2008年   24152篇
  2007年   26829篇
  2006年   12881篇
  2005年   13184篇
  2004年   13092篇
  2003年   13066篇
  2002年   12419篇
  2001年   17950篇
  2000年   17852篇
  1999年   14373篇
  1998年   5328篇
  1997年   5545篇
  1996年   5278篇
  1995年   4946篇
  1994年   4861篇
  1993年   4864篇
  1992年   12436篇
  1991年   12262篇
  1990年   12241篇
  1989年   11985篇
  1988年   11284篇
  1987年   10618篇
  1986年   9892篇
  1985年   10334篇
  1984年   8545篇
  1983年   7369篇
  1982年   5713篇
  1981年   5098篇
  1980年   4763篇
  1979年   8146篇
  1978年   6337篇
  1977年   5982篇
  1976年   5699篇
  1975年   6175篇
  1974年   6723篇
  1973年   6608篇
  1972年   6754篇
  1971年   6141篇
  1970年   4790篇
  1969年   4806篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
31.
It is known that skin releases volatile organic compounds to the environment, and also that its emission pattern changes with aging of the skin. It could be considered, that these compounds are intermediaries in cell metabolism, since many intermediaries of metabolic pathways have a volatile potential. In this work, a simple and non-destructive method consisting of SPME sampling and GC/MS analysis was developed to identify volatile organic emanations from cell cultures. This technique, applied to skin cells culture, indicates that the cells or cell metabolism produce several skin emissions. Chemometric analysis was performed in order to explore the relationship between a volatile profile and the senescence of cell cultures. Volatile profiles were different for cell cultures in different degrees of senescence, indicating that volatile compound patterns could be used to provide information about the age of skin cells.  相似文献   
32.
Abstract Analysis of the Salmonella chromosomal region located upstream of the fimA gene (coding for the major type 1 fimbrial subunit) showed a close linkage of this gene to the folD gene (coding for the enzyme 5,10-methylenetetrahydrofolate dehydrogenase/5, 10-methenyltetrahydrofolate cyclohydrolase), indicating that the fim gene cluster of Salmonella , unlike that of Escherichia coli , has no regulatory genes located upstream of fimA and apparently terminates with this gene. The respective locations of the fim and folD genes in the E. coli and Salmonella genetic maps suggests that the fimA-folD intergenic region of Salmonella encompasses a junctional site of a genetic rearrangement that probably originated from the different chromosomal location of the fim genes in these species.  相似文献   
33.
Hostile intercommunity relations, including attacking and killing extra-community infants of both sexes have occurred at most wild chimpanzee sites. We describe three recent cases of intercommunity attacks on infants committed by members of the Ngogo chimpanzee community in Kibale National Park, Uganda. Two of the attacks resulted in confirmed infanticides while a third attack probably resulted in the infant's death. In common with previous accounts of chimpanzee infanticides, the attacks described here occurred during boundary patrols outside the Ngogo community's usual range, adult and adolescent males were the main participants, one infant was cannibalized after being killed, and the victims’ mothers did not accompany the attacking party back to the Ngogo range. However, the patrol parties during each infanticide were larger than before and included females from the Ngogo community. Our observations indirectly support both the range expansion and imbalance of power hypotheses, which address why and under which conditions chimpanzee intercommunity encounters lead to aggression. These cases of intercommunity infanticide add to the growing database of the phenomenon in wild chimpanzees.  相似文献   
34.
The literature relating to chemical, biochemical and biological aspects of the steroidal glycoalkaloid, α-tomatine, is reviewed. The alkaloid, which can be used as a starting compound for the synthesis of steroidal hormones, is toxic to a wide range of living organisms. The significance of tomatine to plants which elaborate it is discussed and some possible uses of the compound are mentioned.  相似文献   
35.
36.
37.
38.
39.
40.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号